In general terms, a SSAS is a pension scheme for (generally small) employers and a SIPP is a pension scheme for individuals.

However it is useful to further outline and compare the key aspects of both pension arrangements, so we have detailed this below:

SSASSIPP
Occupational pension schemePersonal pension scheme
Employer has over all control of scheme, with Scheme Administrator running day-to-day operationsSIPP provider has control over scheme, SIPP Administrator runs day-to-day operations
Regulated by the Pensions RegulatorRegulated by the FCA
Can loan up to 50% of assets to an employerNo loans to employer permitted
Investments registered in name of trustees

Flexible choice of investments which is decided by member trustees
Investments registered in name of SIPP trustee company.

Investment choice determined by SIPP rules
Can hold shares in an employer and unconnected party (up to 5% of fund value)Can only hold shares in unconnected parties (if allowed by SIPP rules)
Borrowing of 50% of net value allowedAs with SSAS